Organometallic halide perovskites (e.g., CH3NH3PbI3) are a novel class of light absorbing materials that have demonstrated exceptional progress in solar cell performance. But despite rapid progress in device performance, understanding the structural and electronic properties of halide perovskites is just in its infancy and obtaining information on charge diffusion length is critical for the development of perovskite solar cells with balanced light harvesting and charge collection. To address this need, researchers at the National Renewable Energy Laboratory (NREL) have reported on the effect of TiO2 film thickness on charge transport and recombination in solid-state mesostructured perovskite CH3NH3PbI3 solar cells using spiro-MeOTAD as the hole conductor.

Comparison of the electron diffusion length for a mesostructured perovskite solar cell and a solid-state dye-sensitized solar cell using 650-nm TiO2 films.

Intensity-modulated photocurrent/photovoltage spectroscopies show that the transport and recombination properties of solid state mesostructured perovskite solar cells are similar to those of solid-state dye-sensitized solar cells. Charge transport in perovskite cells is dominated by electron conduction within the mesoporous TiO2 network rather than from the perovskite layer. Although no significant film-thickness dependence is found for transport and recombination, the efficiency of perovskite cells increases with TiO2 film thickness from 240 nm to about 650–850 nm, owing primarily to the enhanced light harvesting. Further increasing film thickness reduces cell efficiency associated with decreased fill factor or photocurrent density. The electron diffusion length in mesostructured perovskite cells is longer than 1 micron for over four orders of magnitude of light intensity. The knowledge gained from this study should be valuable for the development of halide perovskite absorbers for solar conversion applications.

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